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connection between discrete microstrip components

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hts_zuo

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when combining two discrete microstrip components with different substrates into one package. if connecting the microstrip port of them by gold strip,not a SMA connector, how to compute the discontinuity and how to improve the match of them?
thanks.
 

This is one application for an electromagnetic simulator or you can also address the question experimentally with a physical (brassboard) model.

Much microwave development can be done with copper tape and a sharp knife.
 

The best is to avoid different substrates. If this is impossible you need to pay great attention to the return path. In any way gold is not good material for high frequencies. It may be used only when connection is exposed and required to stay good for a long time. Electromagnetic simulator will not help you if you simulate just the connections. You need to include the whole path: direct and return with the entire network of related components. This is may be a problem for EM co-simulation.
 

Hi RF-OM
As you have said " Gold is not good material for high frequenies",
Up to what frequency you can use it?
I have heard that people do gold platting on substrate (microstrip circuits) so the performance of circuit becomes better.
Also SMA connectors are Gold plated!!
Then really is it that bad for high frequencies?

I think hts_zuo might be thinking like suppose insted of SMA connectors pin contact which is gold platted suppose we actually use gold then what happens?

You have to simulate that gold plate in simulator to see impedances before actually adding in between two different substrate for contacts.
If you can make the impedance 50 ohm using gold strip then it will be good I suppose.
 

It's not the gold that's bad, but the nickel barrier that's typically between the gold and base metal. Nickel is very lossy. The nickel is what the solder adheres too while the gold dissolves into the solder. Gold is not compatible with tin-lead solder, in fact too much is bad since it will make the joint brittle. When trying to minimize loss, you want to make sure the nickel is below the skin depth on both the bottom and top of the trace. You can go with (almost) pure gold metalization but then that requires lead-indium solder. Check out thin/thick film fabs and you will see all the metalizations offered.
 

To Abhishekabs,
Gold is not good even for DC because it is almost 40% less conductive than copper. Gold plating on RF board without solder mask is actually may kill the board performance because if you try to solder something, for example end-launch, solder will immediately go around the trace and change the metal thickness, roughness and replace the external layer. It will change line impedance and increase loss. Plating gold under solder mask is meaningless. The only reason to use gold is when you need to keep exposed contact in good shape for a long time. This is why gold used for some of the connectors, but not for all and not for very high frequency. SMA is not very high frequency and high quality connector.
Regarding frequency dependence it is skin effect. For example at 4.3 GHz the 90% of electrical current will flow through the outer 1 micron of the metal. If it is gold plated loss will be significantly higher compare to copper. Microwave lines must be cold rolled copper in order to minimize the loss.
 

To RF-OM & madengr.
Thanks for the information
Abhishekabs
 

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